System and method for cardiopulmonary bypass using hypobaric oxygenation

ABSTRACT

A system for cardiopulmonary bypass, including: a cardiopulmonary bypass reservoir configured to store a blood; a pump in fluid communication with the cardiopulmonary bypass reservoir configured to provide pressure to the system; an oxygen source including a pressure regulator configured to regulate an oxygen pressure; an oxygenator fluidly connected to the pressure regulator of the oxygen source via an sweep gas inlet, wherein the sweep gas inlet is configured to have a sub atmospheric pressure and the oxygenator is configured to oxygenate the blood; a vacuum regulator fluidly connected to the oxygenator via an sweep gas outlet, and configured to provide the sub atmospheric pressure; a flow restrictor fluidly connected to the sweep gas inlet and configured to allow for a pressure drop from the oxygen source to the oxygenator; and an arterial filter fluidly connected to a blood outlet of the oxygenator and to the cardiopulmonary bypass reservoir.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national stage application of InternationalApplication No. PCT/US2014/056722 entitled “SYSTEM AND METHOD FORCARDIOPULMONARY BYPASS USING HYPOBARIC OXYGENATION”, which has aninternational filing date of 22 Sep. 2014, and which claims priority toU.S. Provisional Patent Application No. 61/881,684, filed 24 Sep. 2013,the contents of which are incorporated herein by reference in theirentirety.

FIELD

The disclosure relates generally to a system and method forcardiopulmonary bypass, and more particularly to a system and method forcardiopulmonary bypass using hypobaric oxygenation to eliminate gaseousmicroemboli.

BACKGROUND

Primarily during heart operations there is a transient need to replacethe function of the heart and lungs by artificial means. Also in morechronic disease states as e.g. during severe pulmonary, cardiac, orrenal failure, maintenance of life can be upheld by different artificialmeans until an organ for transplantation becomes available. In manyclinical situations there is a need for an extracorporeal circuitwherein the artificial organ is incorporated.

The contact of blood on surfaces made out of foreign material inevitablyinitiates blood coagulation and the formation of clots. This iscontrolled by the use of anticoagulant drugs. Also gas bubbles areeasily formed in blood, which is propelled into the circulation of aliving being during extracorporeal circulation. This phenomenon is dueto cavitation, temperature gradients, and differences in the amount ofgases dissolved between own and incoming blood. In the case of heartsurgery the extracorporeal circuit contains a gas-exchange device i.e.an oxygenator, which is used not only for oxygenation but also for thedisposal of carbon dioxide. The close contact between blood and gas inthe oxygenator poses even higher risks for inadvertent entry of gasbubbles into the circulating blood.

At present, to avoid bubble formation during heart surgery membrane-typeoxygenators are used instead of bubble-oxygenators, high temperaturegradients are avoided, and use of suction in the operating field iscontrolled. Heart-lung machines contain an air bubble sensor that warnsthe perfusionist, i.e. the person maneuvering the heart-lung machine, ofthe appearance of small bubbles and immediately stops the main pump whenlarger bubbles appear. Typically, the bubble sensor can discern bubbleswith a diameter of approximately 0.3 mm, and stops the main pump when abubble with a diameter of 3-5 mm is recognized.

Cardiac surgery is frequently complicated by postoperativeneurocognitive deficits that degrade functional capacity and quality oflife while increasing healthcare costs. Multifactorial contributors tothis significant public health problem likely include gaseousmicroemboli (GME). The arterial circulation receives thousands of 10-40μm GME during cardiopulmonary bypass (CPB) despite the use of membraneoxygenation and arterial filtration. Vasooclusive GME cause tissueischemia and denude endothelium in the brain and other end organs,leading to vascular dilation, increased permeability, activation ofplatelets and clotting cascades, and recruitment of complement andcellular mediators of inflammation.

There are numerous technical solutions in the prior art to separatealready formed bubbles from circulation. Current perfusion practicegenerally targets mildly hyperoxic blood gases during CPB. This targetis achieved by lowering the partial pressure of oxygen in oxygenatorsweep gas by dilution with air, thereby engendering the needless sideeffect of dissolving nitrogen in blood. The blood, thus saturated withdissolved gas, is poorly able to dissolve gases that exist in bubbleform as GME.

However, there is also a need to diminish the generation of gas bubbles,i.e. the formation of gas bubbles during heart surgery, for example. Ina blood bubble, in the liquid-gas interface, there is an approximately40-100 Å (i.e. 4-10 nanometer) deep layer of lipoproteins thatdenaturate due to direct contact with the foreign material, e.g. gas. Inturn, the Hageman factor is activated which initiates coagulation andthe concomitant adverse consumption of factors promoting coagulation,which in the post-pump period are desperately needed to prevent bleedingfrom the surgical wound.

Accordingly, a system and method capable of inhibiting the bubbleformation in the blood in the absence of nitrogen during extracorporealcirculation would be desirable.

SUMMARY

Disclosed is a method and apparatus of hypobaric oxygenation to lowerthe pressure of pure oxygen sweep gas without dilution, thus achievingmildly hyperoxic blood gases in the absence of nitrogen. This approachlowered the sum of partial pressures of dissolved gases tosubatmospheric levels, thereby creating a powerful gradient forreabsorption of GME into the aqueous phase. Both in vitro and in vivoapproaches are utilized to characterize the elimination of GME from CPBcircuits using hypobaric oxygenation, which was accompanied by areduction in dilated brain capillaries in swine.

In an embodiment, a system for cardiopulmonary bypass, including: acardiopulmonary bypass reservoir configured to store a blood; a pump influid communication with the cardiopulmonary bypass reservoir configuredto provide pressure to the system; an oxygen source including a pressureregulator configured to regulate an oxygen pressure; an oxygenatorfluidly connected to the pressure regulator of the oxygen source via answeep gas inlet, wherein the sweep gas inlet is configured to have asubatmospheric pressure and the oxygenator is configured to oxygenatethe blood; a vacuum regulator fluidly connected to the oxygenator via answeep gas outlet, and configured to provide the subatmospheric pressure;a flow restrictor fluidly connected to the sweep gas inlet andconfigured to allow for a pressure drop from the oxygen source to theoxygenator; and an arterial filter fluidly connected to a blood outletof the oxygenator and to the cardiopulmonary bypass reservoir.

A system for cardiopulmonary bypass, including: an oxygen sourceincluding a pressure regulator configured to regulate an oxygenpressure; an oxygenator fluidly connected to the pressure regulator ofthe oxygen source via an sweep gas inlet, wherein the sweep gas inlet isconfigured to have a subatmospheric pressure and the oxygenator isconfigured to oxygenate a blood; a vacuum regulator fluidly connected tothe oxygenator via an sweep gas outlet and configured to provide thesubatmospheric pressure; and a flow restrictor fluidly connected to thesweep gas inlet and configured to allow for a pressure drop from theoxygen source to the oxygenator.

A system for cardiopulmonary bypass, including: a cardiopulmonary bypassreservoir; a pump in fluid communication with the cardiopulmonary bypassreservoir configured to provide pressure to the system; an oxygen sourceincluding a pressure regulator configured to regulate an oxygenpressure; an oxygenator fluidly connected to the pressure regulator ofthe oxygen source via an sweep gas inlet, wherein the sweep gas inlet isconfigured to have a subatmospheric pressure and the oxygenator isconfigured to oxygenate blood; a vacuum regulator fluidly connected tothe oxygenator via an sweep gas outlet, and configured to provide thesubatmospheric pressure; a flow restrictor fluidly connected to thesweep gas inlet and configured to allow for a pressure drop from theoxygen source to the oxygenator; an arterial filter fluidly connected toa blood outlet of the oxygenator configured to filter the blood; and apatient interface fluidly connected to the arterial filter and to thecardiopulmonary bypass reservoir.

A system for cardiopulmonary bypass, including: an oxygen sourceincluding a pressure regulator configured to regulate an oxygenpressure; an air source; a flow control configured to receive the oxygensource and the air source; a vaporizer in fluid communication with theflow control; a sweep gas reservoir in fluid communication with thevaporizer; an oxygenator fluidly connected to a pressure regulator ofthe sweep gas reservoir via an sweep gas inlet, wherein the sweep gasinlet is configured to have a subatmospheric pressure and the oxygenatoris configured oxygenate a blood; a vacuum regulator fluidly connected tothe oxygenator via an sweep gas outlet, and configured to provide thesubatmospheric pressure; and a flow restrictor fluidly connected to thesweep gas inlet and configured to allow for a pressure drop from theoxygen source to the oxygenator.

A method for cardiopulmonary bypass, the method including: providing asubatmospheric pressure in an oxygenator via a vacuum regulator;introducing subatmospheric pressure oxygen to the oxygenator via apressure regulator and a flow restrictor; and introducing blood to beoxygenated to the subatmospheric pressure oxygen.

A system for cardiopulmonary bypass, including: a cardiopulmonary bypassreservoir; a pump in fluid communication with the cardiopulmonary bypassreservoir configured to provide pressure to the system; an oxygen sourceincluding a pressure regulator configured to regulate an oxygenpressure; an oxygenator fluidly connected to the pressure regulator ofthe oxygen source via an sweep gas inlet, and configured to receiveblood from the cardiopulmonary bypass reservoir wherein the sweep gasinlet is configured to have a subatmospheric pressure and the oxygenatoris configured to oxygenate blood; a patient reservoir in fluidcommunication with a blood outlet of the oxygenator; a second pump influid communication with the patient reservoir configured toadditionally provide pressure to the system; and a patient simulatorconfigured to introduce carbon dioxide into the blood and remove oxygenfrom the blood, wherein the patient simulator is fluidly connected to acardiopulmonary bypass reservoir.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying drawings incorporated in and forming a part of thespecification embodies several aspects of the present disclosure and,together with the description, serves to explain the principles of thisdisclosure. In the drawings:

FIG. 1A is a schematic representation of an embodiment of a hypobaricoxygenation apparatus;

FIG. 1B is a schematic representation of an embodiment of an in vitrogas exchange circuit;

FIG. 1C is a chart of O₂ tension (millimeters of mercury, mmHg) versussweep gas pressure (atmospheres, ata) illustrating the relationshipbetween oxygenation and sweep gas pressure;

FIG. 1D is a chart of CO₂ tension (millimeters of mercury, mmHg) versussweep gas pressure (total ambient pressure, ata) illustrating therelationship between carbon dioxide removal and sweep gas pressure;

FIG. 2A is a schematic representation of an embodiment of a singleoxygenator CPB circuit configuration;

FIG. 2B is a chart of doppler signal (arbitrary units) versus time(seconds) illustrating the relationship between doppler signal and timewhen sweep gas pressure is varied;

FIG. 2C is a chart of doppler signal (arbitrary units) versus time(seconds) illustrating the relationship between doppler signal and timeupstream of the arterial filter when sweep gas pressure is varied;

FIG. 2D is a chart of doppler signal (arbitrary units) versus time(seconds) illustrating the relationship between doppler signal and timedownstream of the arterial filter when sweep gas pressure is varied;

FIG. 3A is a schematic representation of an embodiment of a CPB circuitwith Emboli Detection and Classification (EDAC) and Doppler monitors;

FIG. 3B is a series of histograms of gaseous microemboli per minuteversus gaseous microemboli size (micrometers) showing EDAC GME counts;

FIG. 4A is a 10× photomicrograph of 4 μm-thick hematoxylin-eosin;

FIG. 4B is a chart of dilated capillaries illustrating the relationshipbetween dilated capillaries and hypobaric oxygenation;

FIG. 4C is a chart of capillary area (square micrometers) illustratingthe relationship between capillary area and hypobaric oxygenation;

FIG. 4D is a chart of dilated capillaries versus capillary diameter(micrometers, μm) illustrating the relationship between dilatedcapillaries and hypobaric oxygenation with respect to capillarydiameter;

FIG. 5A are blood photomicrograph samples taken before initiation of CPB(baseline), then after two and four hours of CPB with hypobaricoxygenation;

FIG. 5B are photomicrograph samples of Hematoxylin-eosin stained,paraffin-embedded, 4 μm thick sections from cerebral cortex;

FIG. 5C are photomicrograph samples of hippocampal region CA1;

FIG. 5D are photomicrograph samples of renal cortex; and

FIG. 6 is a schematic representation of a hypobaric oxygenation methodin accordance with an exemplary embodiment.

DETAILED DESCRIPTION

The following disclosure will detail particular embodiments, whichprovides methods and systems for cardiopulmonary bypass. An overview ofthe mechanism and methods used herein is provided.

Hypobaric oxygenation controls the oxygenator's gas:blood O₂ diffusiongradient to achieve desired blood gases without using nitrogen. Theresultant decrease in dissolved blood gases favors aqueous reabsorptionof GME, resulting in enhanced GME removal observed throughout the CPBcircuit. The magnitudes of the observed effects on GME seem consistentwith published dynamics of air microparticles in undersaturated aqueoussolutions. Of note, blood gas undersaturation is more important thandenitrogenation alone. In vitro data demonstrate the difference betweena denitrogenated normobaric oxygen control condition and adenitrogenated/undersaturated hypobaric oxygenation condition. Anadditional physical effect of subatmospheric pressures within theoxygenator's hollow fibers may contribute to oxygenator GME removal, butnot to GME removal at more distal sites.

As the beneficial effect of hypobaric oxygenation on GME continues asblood flows into the patient, additional benefits may be realized on airentrained from the surgical field into the arterial circulation duringopen-heart procedures. Hypobaric oxygenation should also ameliorateconcerns about increased GME delivery due to pulsatile flow, centrifugalpump cavitation, and outgassing during vacuum-assisted venous drainageor with rapid temperature changes.

The reduction in brain microvascular injury in animals managed withhypobaric oxygenation suggests improved end-organ function followingCPB. While hypobaric oxygenation practically eliminated GME delivery,capillary dilations were only partially reduced. Recycled mediastinalshed blood used to maintain the animals' hematocrit may also increaselipid embolization and may account for residual microvascular damageseen in the hypobaric condition.

Hypobaric oxygenation does not change CPB circuit priming volumes,material composition, or ease of use. The perfusionist controls PaO₂ byadjusting the pressure of pure oxygen sweep gas rather than adjustingthe sweep gas oxygen content, while PaCO₂ is still adjusted by varyingthe sweep gas flow rate. As partial pressures of anesthetic vapors arealso reduced in proportion to the sweep gas pressure, an adjustment ofanesthetic concentration will be necessary to ensure adequateanesthesia. As the oxygenator housing must be sealed in order to applysubatmospheric pressures, a suitable pressure relief system must existto prevent gross air embolism in the event of occlusion of the sweep gasoutlet or vacuum failure. Application of overly negative sweep gaspressures could result in hemoglobin desaturation, the solution forwhich would be to increase the sweep gas pressure or disconnect thevacuum source. Hypobaric oxygenation should be used along with, ratherthan instead of, arterial filtration in the CPB circuit. Among otherbenefits, arterial filtration reduces the size of GME, therebyincreasing the surface-to-volume ratio and promoting rapid reabsorptionunder conditions of hypobaric oxygenation.

Reference is now made to the drawings, wherein like reference numeralsare used to refer to like elements throughout the disclosure.

FIGS. 1A-1D relate to hypobaric oxygenation in vitro. FIG. 1A refers toa hypobaric oxygenation apparatus. Pure oxygen is supplied to the sweepgas inlet of a standard hollow fiber microporous membrane oxygenatorwith a sealed housing. Alternatively, the oxygenator may not containmicropores. The housing of the oxygenator may be sealed by physicallyoccluding the vents of a commercially available oxygenator housing,including physically occluding the vents with epoxy putty. Similarly,any suitable method may be used to obtain a sealed housing for theoxygenator. A regulated vacuum source at the sweep gas outlet appliesuser-determined variable subatmospheric pressure to the sweep gascompartment to regulate the partial pressure gradient for bloodoxygenation. A vacuum gauge measures the pressure applied, while apositive-pressure relief valve (PPR) insures against creation ofpositive pressures. A flowmeter, e.g., a needle-valve flowmeter, at thesweep gas inlet regulates sweep gas flow rate and thus CO₂ removal,while allowing a pressure drop from ambient to subatmospheric.

FIG. 1B refers to an in vitro gas exchange circuit. A mixture of humanred blood cells (RBCs), fresh frozen plasma (FFP), and minimalcrystalloid (Hct˜30%) from a CPB reservoir is pumped (3.5 liters/minute)to the CPB oxygenator (37° C.), where oxygenation occurs with pureoxygen sweep gas at variable subatmospheric pressure. The blood thenpasses into a Patient Simulator consisting of a reservoir, pump, andoxygenator that removes O₂ and adds CO₂ using pure CO₂ sweep gas at verylow pressure (1 liter/minute, 0.1 ata). Blood gases were sampleddownstream of the CPB oxygenator (arterial) and the Patient Simulator(venous, n=3 samples per condition). The simulated patient producednormal venous blood gas values.

To assess the effect of sweep gas pressure on blood pressure at theoxygenator outlet, paired measurements were performed at sweep gaspressures of ambient, 0.5 atmospheres (ata), and 0.1 ata (n=14 trialsper condition). Blood pressure was measured using a pressure transducer(ICU Medical, San Clemente, Calif.) connected to a demodulator (ValidyneCorporation, Northridge, Calif.), whose voltage output was calibratedagainst a water column, digitized (DI-145) and recorded using Windaqsoftware (DATAQ Instruments, Akron, Ohio).

FIGS. 1C and 1D illustrate that the application of subatmospheric sweepgas pressure in the CPB oxygenator reduced arterial oxygenation in theexpected linear manner independent of CO₂ removal.

FIGS. 2A-2D further relate to hypobaric oxygenation in vitro.Specifically, hypobaric oxygenation greatly enhances GME removal invitro. FIG. 2A illustrates a single oxygenator CPB circuit configurationwith arterial filter and purge line returning to reservoir. Locations ofair introduction and Doppler GME monitoring are shown. Arrow indicatesdirection of blood flow (5 liters/minute, 37° C.). The Doppler signalwas processed via custom analog envelope detector then digitized andrecorded as above.

FIG. 2B is a chart illustrating the relationship and effectiveness ofsubatmospheric sweep gas. A tiny (˜20 μl) air bubble agitated by hand in10 ml blood (using two 12 ml syringes and a 3-way stopcock) was injectedupstream of the oxygenator. The emboli traversed the oxygenator and weredetected proximal to the arterial filter. Application of slightlysubatmospheric sweep gas pressures strongly attenuated the GME Dopplersignal. Data are averages of 9-10 trials per condition.

FIGS. 2C and 2D further illustrate the relationship and effectiveness ofsubatmosphereic sweep gas before and after the arterial filter.Continuous entrainment of air (500 ml/minute, via luer connector) intothe venous line at the reservoir entrance simulated a large, continuousembolic challenge. With ambient sweep gas pressures, strong Dopplersignals were observed upstream and downstream of the arterial filter.Hypobaric oxygenation produced a robust dose-dependent reduction of theDoppler signal in both locations. Data are averages of 3 continuoustrials that employed 2-minute steps to each listed pressure level.

FIGS. 3A and 3B relate to hypobaric oxygenation in vivo. Specifically,hypobaric oxygenation nearly eliminates GME delivery in vivo. FIG. 3Aillustrates a CPB circuit with EDAC and Doppler monitors and venous airentrainment site (200 ml/minute).

FIG. 3B shows histograms showing EDAC GME counts and sizes at the fourmonitoring sites for both control (O₂/air sweep gas at ambient pressure)and hypobaric (O₂ sweep gas at subatmospheric pressure) conditions.Under control conditions ˜4500 GME/minute were delivered to the patient.Under hypobaric conditions, GME counts and volumes were similar tocontrol at the preoxygenator location, but were progressively eliminatedas they traversed the CPB circuit, reducing GME delivery to only2/minute during this large embolic load.

FIGS. 4A-4D relate to animals managed with hypobaric oxygenation.Specifically, animals managed with hypobaric oxygenation display reducedmicrovascular injury in cerebral white matter.

FIG. 4A shows 10× photomicrographs of 4 μm-thick hematoxylin-eosinstained sections of periventricular white matter. Dilated capillariesappear as voids (white) surrounded by a single layer of endothelialcells. Scale bars=100 μm.

FIGS. 4B and 4C illustrate that dilated capillaries were fewer in numberand in area in animals managed using hypobaric oxygenation (control n=3010×-fields, N=3 animals; hypobaric n=51 fields, N=5 animals; *=p<0.001).

Further, FIG. 4D illustrates that the difference in numbers of dilatedcapillaries between conditions was present at all sizes studied, toensure that the result is not an artifact of the measurement criteria.

FIG. 6 refers to a schematic illustration of an alternative embodimentof the CPB system. Oxygen and/or air may be introduced at inletcomprising a gas blender 1 from an oxygen source equipped with apressure regulator configured to control a pressure of the suppliedoxygen, e.g., to control the volume fraction of inspired oxygen(F_(i)O₂), and wherein a flow is controlled by flow control 2. The flowcontrol can be a needle-valve flow controller, for example. Vaporizer 3,negative pressure alarm 4, sweep gas reservoir 5, pressure meter 6, andpressure relief 7 all operate at or near ambient pressure. Pressurerelief 7 may be configured to relieve pressure in the sweep gasreservoir 5 if pressure exceeds 50 mmHg. Sweep gas reservoir 5 storesthe sweep gas to be used in the oxygenation process at ambient pressure.The sweep gas reservoir 5 may be elastic. Flow restrictor 8 limits theflow of sweep gas (oxygen) into the oxygenator 9. Oxygenator 9experiences low pressure due to vacuum imparted by vacuum source 14.Vacuum source 14 is regulated by vacuum regulator 13, and is measured byvacuum meter 11. In case of failure, manual open valve 12 may beutilized. Positive pressure relief 10 and positive pressure alarm may beutilized if subatmospheric pressure is not obtained. Pressure relief 10may relieve pressure if pressure exceeds a given range of 0-15 mmHg.Flow restrictor 8, oxygenator 9, pressure relief 10, vacuum meter 11,manual open 12, vacuum regulator 13, and vacuum source 14 are held atsubatmospheric pressure.

Additionally, a control panel may be used to control FiO₂, sweep flowrate, vacuum level and the desired anesthetic concentration. Further,the control panel can output actual values, alarms, blender/flow controland anesthetic compensation. Sweep flow rate may open flow control 2 toa desired flow rate, and then adjust the flow restrictor 8 to maintainconstant pressure in the sweep gas reservoir 5. Preferably, thispressure may be slightly above atmospheric, in at least one embodiment,may be 20 mmHg. The control panel may monitor the pressure meter 6. Thecontrol panel may adjust the vacuum level by monitoring the vacuum meter11 and adjusting the vacuum regulator 13 accordingly.

Further, the desired anesthetic concentration in the system may becontrolled, with various modes of operation and control methods. Severalmethods are contemplated herein; however the desired anestheticconcentration may be achieved by any suitable method. First, theadjustment may be made by indexing the vaporizer knob on vaporizer 3 foran assortment of vacuum levels, wherein the perfusionist adjusts thelevels appropriately. Secondly, the adjustment may be made based thereading of vacuum meter 11, therefore adjusting the vaporizer 3 bypassratio. Thirdly, the sweep gas reservoir 5 and vaporizer 3 may be exposedto subatmospheric pressures to increase vaporizer output, wherein thesubatmospheric pressure may be adjusted based on the reading of vacuummeter 11. Lastly, the vaporizer may be set to a certain predeterminedoutput, ranging preferably, but no limited to 4-8%, wherein output ismixed with a fresh O₂ source between the vaporizer 3 and the sweep gasreservoir 5.

Examples

Eight juvenile swine were studied. Anesthetic induction employedintramuscular acepromazine (1.1 mg/kg), glycopyrrolate (0.01 mg/kg), andketamine (33 mg/kg), followed by 3% isoflurane inhalation withelectrocardiography and pulse oximetry. Fentanyl (50 mcg in 5 ml normalsaline) was administered into the lumbar cerebrospinal fluid via20-gauge multiorifice catheter. Tracheal intubation and ear veincannulation were followed by isoflurane maintenance in 70% O₂/30% N₂ and50 mcg of additional spinal fentanyl when warranted by hemodynamics.After median sternotomy, a central arterial pressure catheter was placedand heparin was administered IV to achieve and maintain ACT>350 seconds(Hemochron Response, International Technidyne Corporation, Edison,N.J.). A 20-french aortic and 28-30-french bicaval CPB cannulae(Medtronic Incorporated, Minneapolis, Minn.) were placed.

Animals were assigned a priori to control or hypobaric oxygenation usinga single-oxygenator, filtered CPB circuit (FIG. 3A). M3 Detector(Spectrum Medical, Cheltenham, UK) continuously monitored flow rates andarterial/venous O₂ saturations (SaO₂/SvO₂). PaO₂ was adjusted(target=200 mmHg) by varying the sweep gas oxygen/air mixture (controlconditions, N=3 animals) or by applying variable subatmospheric pressureto 100% O₂ sweep gas (hypobaric oxygenation, N=5 animals). PaCO₂(target=45 mmHg) was adjusted by varying the sweep gas flow rate in eachcondition. In the hypobaric condition, the predicted partial pressure ofisoflurane was maintained by increasing the vaporizer dial setting toequal the desired concentration/sweep gas pressure (e.g. if 1%isoflurane were used at ambient pressure, the setting would need to beincreased to 1.5% at 0.66 atmospheres absolute (ata) and to 2% at 0.5ata to achieve an equal hypnotic effect). CPB flow rates were adjustedto maintain SvO₂>60%, and intermittent phenylephrine maintained arterialpressure (MAP)>50 mmHg. Reservoir volume was 250-500 ml. To conserve theanimals' low starting hematocrit, mediastinal shed blood was returned tothe reservoir's cardiotomy section via ¼-inch roller pump circuit.Vacuum-assisted venous drainage was used (−10 mmHg). Passive cooling wasallowed to 34° C.

Air was continuously entrained throughout the CPB run (200 ml/minute,via luer connector in venous line at reservoir entrance, FIG. 3A).Single-site Doppler monitored GME semi-quantitatively in all 8experiments. Terumo's FDA-approved Emboli Detection and Classification(EDAC) was available in 6 of 8 experiments for simultaneous 3-siteultrasound backscatter GME quantification: at preoxygenator, immediatelypostfilter, and either postoxygenator (N=3 animals) or 6 feet postfilter(N=3) locations. GME data were acquired in trials of 10-120 minutesduration separated by a change in CPB conditions (e.g. flow rate,reservoir volume, sweep gas composition or pressure). Brief trials ofhypobaric oxygenation were performed in 2 of 3 animals otherwise managedwith control conditions. Thus, hypobaric data are from N=⅞ animals forDoppler and N=⅚ animals for EDAC, while all control data are from N=3animals (number of trials (n) listed in Results). As CPB parameters arelikely larger determinants of microembolization than the animaldownstream of the CPB circuit, GME data trials were considered to beindependent observations for statistical analysis.

Hematologic samples taken before CPB, then after two and four hours ofCPB, were analyzed for RBC morphology and plasma hemoglobin. End organswere fixed with neutral buffered formalin (10%, 3 liters, 5 minutes) viaCPB circuit before harvest.

Paraffin-embedded, 4 μm hematoxylin-eosin sections from the frontallobe, thalamus, caudal lobe, mesencephalon, cerebellum, medulla, andrenal cortex were evaluated microscopically by veterinary pathologistsfor cytoarchitectural integrity. Then, blinded quantification of dilatedcapillaries (>10 μm diameter) was performed in white matter adjacent tothe lateral ventricle and subependymal zone (periventricular whitematter, ˜10 10× fields per animal). Due to the expected heterogeneousnature of tissue effects and the exploratory nature of the post-hocmicrovascular analysis, we treated each field as an independent datapoint with respect to microvascular injury.

Data are presented as mean±SEM. Continuous variables were compared usingtwo-tailed Student's t-tests (significance at P<0.05). A linear fit wasperformed using Prism (GraphPad Software, La Jolla, Calif.).Dose-dependence was assessed using Spearman's Rank CorrelationCoefficient. GME data trials and tissue specimens were treated asindependent observations for statistical analyses.

In Vitro Gas Exchange: Reduction of Dissolved Gases in Blood

TABLE 1 In vitro Gas Exchange O₂ Tension CO₂ tension (mmHg) (mmHg) # oftrials Arterial Blood Gases Ambient sweep gas pressure 527 ± 12 44.6 ±4.9 3 0.8 ata 359 ± 7  40.2 ± 2.3 3 0.6 ata 205 ± 6  42.0 ± 4.2 3 0.4ata 96 ± 4 30.3 ± 2.3 3 Venous Blood Gases Ambient sweep gas pressure63.0 ± 3.8 52.4 ± 5.5 3 0.8 ata 61.3 ± 2.3 48.0 ± 3.8 3 0.6 ata 61.0 ±2.0 56.3 ± 6.0 3 0.4 ata 59.3 ± 0.3 52.7 ± 2.4 3

Hypobaric oxygenation (FIG. 1A) was used with a simulated patient on CPB(FIG. 1B) to assess the effect of subatmospheric sweep gas pressures onoxygenation and CO₂ removal from blood in the absence of nitrogen. Asexpected, lowering the pressure of pure oxygen sweep gas decreased PaO₂in a smooth, linear manner (Table 1, FIG. 1C, R²=0.99). In contrast,PaCO₂ was largely stable with decreasing sweep gas pressures, withpossibly increased efficiency of CO₂ removal at the lowest pressuresapplied (Table 1, FIG. 1D). Indeed, CO₂ removal was easily managedindependently of oxygenation by adjusting the sweep gas flow rate inexperiments. Hypobaric oxygenation was confirmed to reduce dissolvednitrogen in blood by 85.4±0.7% using mass spectroscopy (n=3 trials,p<0.001, denitrogenation likely underestimated, see supplementalinformation). No detrimental effects on the oxygenator or gas exchangewere observed in any experiments. During simulated vacuum failure, thepositive-pressure relief valve successfully prevented air embolism (datanot shown). Together, these data indicate that hypobaric oxygenation isa reliable, efficient method for managing gas exchange during CPB thatreduces the sum of partial pressures of dissolved gases tosubatmospheric levels.

In Vitro GME: Dose-Dependent Removal

Next tested was whether hypobaric oxygenation improves GME removal inthe CPB circuit. First, GME boluses were injected upstream of theoxygenator (FIG. 2A). When the oxygenator used 100% oxygen sweep gas atambient pressure, a robust downstream Doppler signal was observed (FIG.2B). Modest reduction of sweep gas pressure to 0.9 ata reduced theDoppler signal by 94.8±1.0% from control (area under curve, range90.9-98.3%, n=9 trials, p<0.001), while at 0.8 ata the signal was barelydiscernible (reduced 99.6±0.07% from control, range 99.3-99.8%, n=10trials, p<0.001). Next, air entrainment simulated a large ongoingembolic load. Reductions in sweep gas pressure again produced a robustdose-dependent reduction of the Doppler signal measured before thearterial filter (FIG. 2C, reduced by 26±3% when the sweep gas pressurewas 0.9 ata, 66±2% at 0.8 ata, 83±2% at 0.7 ata, 91±0.2% at 0.6 ata,95±2% at 0.5 ata, and 98±1% at 0.4 ata, Spearman's rank correlationcoefficient ρ=1.0, n=3 continuous trials). When the Doppler was moveddownstream of the arterial filter, a substantial embolic signalpersisted under normobaric conditions despite the use of arterialfiltration (81±3% of pre-filter baseline signal, FIG. 2D). Thedownstream signal was reduced even more effectively by reductions insweep gas pressure (by 53±4% of post-filter baseline at 0.9 ata, 83±2%at 0.8 ata, 94±1% at 0.7 ata, 99±0.3% at 0.6 ata, 99.5±0.1% at 0.5 ata,and 99.7±0.1% at 0.4 ata, ρ=1.0, n=3 continuous trials). Together, thesedata demonstrate that the use of hypobaric oxygenation improves theability of the CPB circuit to remove GME from circulating blood in adose-dependent fashion.

Swine CPB: Safe Maintenance of Large Animals

TABLE 2 Swine Characteristics Control Hypobaric P value SwineCharacteristics Gender 2M/1F 4M/1F Mass (kg) 43.5 ± 1.8 42.3 ± 1.5 0.64Hematocrit_(start) (%) 21.3 ± 1.2 22.2 ± 1.0 0.61 Hematocrit_(end) (%)18.3 ± 0.3 18.8 ± 0.4 0.43 Induction to CPB (minutes) 168 ± 10 176 ± 8 0.58 Blood Gases pH  7.30 ± 0.06  7.40 ± 0.01 0.06 P_(a)CO₂ (mmHg) 52.1± 2.0 45.0 ± 1.3 0.02 P_(a)O₂ (mmHg) 190 ± 14 184 ± 14 0.81 S_(a)O₂ (%)99.6 ± 0.4 99.6 ± 0.2 0.91 S_(v)O₂ (%) 66.2 ± 1.0 66.7 ± 2.4 0.88 BypassParameters Sweep pressure (ata) 1 (ambient)  0.66 ± 0.03 Sweep F_(i)O₂(%)  0.68 ± 0.02 1.0 MAP (mmHg) 51.0 ± 1.6 52.2 ± 1.8 0.68 Flow Rate(liters/minute)  4.25 ± 0.14  4.04 ± 0.21 0.36 Pump Time (hours)  4.57 ±0.23  4.52 ± 0.14 0.86

CPB in 40 kg swine using hypobaric oxygenation was characterized bystable, easily adjustable gas exchange parameters with no adverseeffects noted in the animal, oxygenator, or CPB circuit. Animalcharacteristics and CPB management data are listed in Table 2. Notably,lowering the partial pressure of oxygen in the sweep gas using dilutionwith nitrogen (F_(i)O₂=68.3±1.7% in control) or using vacuum(pressure=0.66±0.03 ata in hypobaric conditions) produced similar PaO₂values, suggesting that hypobaric oxygenation preserves oxygenator gasexchange efficiency. A small increase in CO₂ tension in control animalswas due to a slightly lower sweep gas flow rate in the absence ofvacuum. Overall, hypobaric oxygenation was a reliable, practical methodfor maintaining large animals during CPB.

Swine GME: Progressive Elimination in the CPB Circuit

During continuous air entrainment, single-site semi-quantitative Dopplerand multisite quantitative EDAC detected GME in the CPB circuit (FIG.3A). Data were collected in discrete trials (n) separated by a change inCPB parameters in N=8 total swine. Hypobaric oxygenation reduced theDoppler signal near the aortic cannula by 99.1±0.3% (range 95.8-100%,n=16 trials, N=7 swine) compared with control (n=7, N=3). EDAC data fromhypobaric (N=5 swine) and control (N=3) conditions are presented in FIG.3B. Proximal to the oxygenator, EDAC GME counts and volumes were similarbetween hypobaric (n=10 trials, p>0.29) and control (n=7). Downstreamfrom the oxygenator, GME numbers in the hypobaric condition were greatlyreduced compared with controls: at the post-oxygenator location by68.4±14.1% (range 13.1-91.3%, n=5, p<0.05), at the post-arterial filterlocation by 92.6±4.2% (range 57.8-99.8%, n=10, p<0.01), and at theentering patient location by 99.96±0.02% (range 99.87-99.99%, n=5,p<0.001). GME volumes were also reduced at the post-oxygenator locationby 80.5±5.6% (range 62.2-95.6%, n=5, p<0.05), at the post-filterlocation by 94.9±2.9% (range 74.3-99.8%, n=10, p<0.01), and at theentering patient location by 99.97±0.01% (range 99.94-100%, n=5,p<0.001). Control trials numbered n=3, 7, and 4 in these respectivelocations. These data demonstrate that hypobaric oxygenation stronglyenhances GME removal from the CPB circuit in vivo. Further, theyindicate that the site of enhanced GME removal extends progressivelyfrom the oxygenator to the arterial filter to the blood itself as itflows toward the patient Importantly, data acquired closest to thepatient show near-total elimination of GME delivery from the CPB circuitby the combined use of arterial filtration and hypobaric oxygenation,despite a large upstream embolic load.

Swine Tissue Analysis: Normal Histology with Reduced MicrovascularDamage

FIGS. 5A-5D illustrate microscopic evaluation of peripheral blood,brain, and kidney from animals managed with hypobaric oxygenation (Scalebars=100 μm). Specifically, these figures revealed normalcytoarchitecture.

One skilled in the art would expect that an oxygenator usingnitrogen-free sweep gas at any pressure would create a maximal gradientfor N₂ removal, thereby denitrogenating blood that flows through it,even if the sum of partial pressures of the remaining O₂ and CO₂ in theblood totaled less than the ambient pressure. However, since theproposed mechanism for reabsorbing air bubbles into the blood phasedepends critically on reducing the sum of partial pressures of dissolvedgases, it was optimal to confirm experimentally that N₂ tension wasindeed reduced during hypobaric oxygenation. As N₂ tension is notmeasured by standard blood gas analysis, mass spectrometry was utilized.The analysis employed a single oxygenator CPB circuit where O₂ bubblescould be injected into a flowing blood stream, then made to exchangegases with that blood, then recollected in a bubble trap and sampled.Reasonably, the dissolved N₂ in blood would accumulate in the O₂ bubblesduring their passage, and would then be detected by mass spectrometry.Oxygen (20-30 ml) was agitated in blood and injected slowly into thecircuit downstream of the oxygenator, where it was made to flow throughthe pores of an arterial filter to enhance exchange of gases with theblood, then collected in a bubble trap and sampled at ambient pressureusing a 100 μl gaslock syringe (Supelco Analytical). Mass spectrometry(Agilent 5975C GC-MS) was used to analyze control samples of 100% O₂ androom air, then the test sample gas was introduced in volumes of 20-100μl. The gas mixtures were injected and run through a HP-5MS column withhelium gas flow controlled at 1 ml/min. The mass spectrometer ionsource, column and quadrupole were maintained at 50° C. The massspectrometer was used in the electron impact mode and the mass range forion acquisition was 14 to 200 atomic mass units (amu). Molecular ions ofO₂ and N₂ were monitored by their respective molecular ions ofmass-to-charge (m/z) 32 and m/z 28. The peak ion abundances for m/z 28(N₂) and m/z 32 (O₂) amu were measured, and an O₂/N₂ fraction wascalculated by interpolation between the O₂ and air calibration samples.

Blood was oxygenated in a single-oxygenator circuit using a sweep gasmixture of 50% O₂/50% N₂ at ambient pressure to produce blood containingdissolved N₂. When O₂ bubbles were passed through this blood, thencollected and analyzed they had accumulated 41.7+/−2.3% N₂ by mass (n=7trials). The same blood was oxygenated with 100% O₂ sweep gas at 0.5atmospheres absolute (ata) to produce a similar moderate level ofoxygenation in the absence of N₂. O₂ bubbles that were passed throughthis blood accumulated significantly less N₂ (6.1+/−0.3%, n=3 trials,p<0.001). The magnitude of the denitrogenation is likely underestimatedby these data, as there were several opportunities for contamination ofthe O₂ samples with room air during the processes of injection,sampling, and measurement. The data indicate a clear reduction of N₂tension during hypobaric oxygenation, consistent with the proposedmechanism of GME reabsorption through reduction of the sum of partialpressures of dissolved gases in blood.

FIG. 5A illustrates peripheral blood smears were made from samples takenbefore initiation of CPB (baseline), then after two and four hours ofCPB with hypobaric oxygenation. At 1000× magnification, smears displayred blood cell echinocytes with a crenated appearance, which is typicalfor swine, but no evidence of cellular injury or hemolysis. FIG. 5Billustrates Hematoxylin-eosin stained, paraffin-embedded, 4 μm thicksections from cerebral cortex without apparent abnormality. FIG. 5Cillustrates hippocampal region CA1 without apparent abnormality. FIG. 5Dillustrates renal cortex without apparent abnormality.

Much like the human lung, hollow fiber microporous membrane oxygenatorsare very efficient at exchanging O₂ and CO₂ between sweep gas and bloodwhen the sweep gas flow rate is adequate. The human lung more rapidlyequilibrates the partial pressures of gases or vapors (e.g. anestheticvapors) with lower blood solubility compared with those of greater bloodsolubility. Similarly, it is expected the addition or removal of N₂ bythe oxygenator to be more efficient than the exchange of O₂ or CO₂ dueto its lower solubility in blood. As known in the art, the partialpressure of N₂ in blood exiting the oxygenator approaches zero when thesweep gas does not contain N₂, a simple sum of the measured partialpressures of O₂ and CO₂ in Table 1 and the hypobaric column of Table 2should provide reasonable estimates of the sum of partial pressures ofdissolved gases in experiments. For the Control column of Table 2, thepartial pressure of N₂ in the sweep gas would also need to be added toestimate the sum of partial pressures of dissolved gases.

Since the washout of N₂ from the body is somewhat prolonged due to theslowly mobilized stores of N₂ in fat, the swine in the experiments areexpected to serve as sources of N₂ in the venous blood. As themeasurement of dissolved N₂ in blood is difficult and would beimpractical in a clinical situation, one skilled in the art wouldprovide estimates of N₂ elimination from the swine. Anesthetizedpatients before CPB initiation are frequently exposed to inspired O₂ inthe 60-100% range, and thus are partially denitrogenated before CPB.Based on a published human denitrogenation timecourse, one skilled inthe art would estimate that the swine have 60% equilibrated to the 70%O₂/30% N₂ ventilator gas during the 176+/−8 minutes from anestheticinduction to initiation of CPB in our hypobaric experiments. Using thisestimate to modify a published 4-hour N₂ elimination timecourse from37-kg swine previously equilibrated to air breathing, it is estimatedthe N₂ elimination from a 42.3-kg swine to be 4.8 ml/minute during thefirst 7 minutes of CPB using N₂-free sweep gas, 1.9 ml/minute at 1 hour,and 0.5 ml/minute at 4 hours. Using the published N₂ solubility in humanblood at body temperature of 1.27 ml N₂ per 100 ml blood and the meanCPB flow rate of 4 liters/minute, the eliminated nitrogen accounts for avenous partial pressure of dissolved N₂ from the animal of 68 mmHgduring the first 7 minutes, 23 mmHg at 1 hour, and 8 mmHg at 4 hours ofCPB.

As the cylindrical hollow fiber microporous oxygenator membranes are notexpected to transfer pressures from the sweep gas compartment into theblood phase, it is not expected for blood cells to experience hypobaricpressures. Indeed, subatmospheric sweep gas pressures did not affectarterial line blood pressures at the oxygenator outlet (differences fromcontrol ranged from −0.8 to +0.6 mmHg at 0.5 ata (n=14 trials, p>0.9)and from −0.6 to +0.5 mmHg at 0.1 ata (n=14 trials, p>0.4).Additionally, no plasma hemoglobin or morphologic evidence of hemolysiswas observed in blood samples taken before and during hypobaricoxygenation in swine (FIG. 5A).

Experienced veterinary pathologists generally found no cytoarchitecturalabnormality or apparent difference between animals in fixed postmortemtissue from six brain regions and the renal cortex (FIGS. 5B-5D).However, the presence of abnormal dilated capillaries in some whitematter regions was noted. Since small capillary and arteriolar dilationsare known responses to microembolization. Blinded post-hoc analysis inperiventricular white matter revealed that reduction of GME duringhypobaric oxygenation was accompanied by 35.8±5.5% and 48.7±5.7%reductions in the number and area of dilated capillaries, respectively(FIG. 4, p<0.001) when compared with control. Similar findings wereobserved at all capillary diameters studied (FIG. 4D). Collectively,these data suggest that elimination of GME is accompanied by reducedmicrovascular injury in animals maintained using hypobaric oxygenation.

All references, including publications, patent applications, and patentscited herein are hereby incorporated by reference to the same extent asif each reference were individually and specifically indicated to beincorporated by reference and were set forth in its entirety herein.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) is to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Exemplary embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those embodiments may become apparent to those of ordinaryskill in the art upon reading the foregoing description. The inventorsexpect skilled artisans to employ such variations as appropriate, andthe inventors intend for the invention to be practiced otherwise than asspecifically described herein. Accordingly, this invention includes allmodifications and equivalents of the subject matter recited in theclaims appended hereto as permitted by applicable law. Moreover, anycombination of the above-described elements in all possible variationsthereof is encompassed by the invention unless otherwise indicatedherein or otherwise clearly contradicted by context.

What is claimed is:
 1. A system for cardiopulmonary bypass, comprising:a cardiopulmonary bypass reservoir configured to store a blood; a pumpin fluid communication with the cardiopulmonary bypass reservoirconfigured to provide pressure to the system; an oxygen sourcecomprising a pressure regulator configured to regulate an oxygenpressure; an oxygenator fluidly connected to the pressure regulator ofthe oxygen source via a sweep gas inlet, wherein the sweep gas inlet isconfigured to have a subatmospheric pressure and the oxygenator isconfigured to oxygenate the blood; a vacuum regulator fluidly connectedto the oxygenator via a sweep gas outlet, and configured to provide thesubatmospheric pressure; a vaporizer in fluid communication with theoxygenator; a flow restrictor fluidly connected to the sweep gas inletand configured to allow for a pressure drop from the oxygen source tothe oxygenator, and disposed between the vaporizer and the oxygenator;and an arterial filter fluidly connected to a blood outlet of theoxygenator and to the cardiopulmonary bypass reservoir.
 2. The system ofclaim 1, further comprising a positive pressure relief valve fluidlyconnected to the sweep gas outlet.
 3. The system of claim 1, wherein theoxygenator is a microporous membrane oxygenator having a sealed housing.4. The system of claim 1, wherein the arterial filter is fluidlyconnected to the cardiopulmonary bypass reservoir via a purge line. 5.The system of claim 1, further comprising a first Doppler site in fluidcommunication with the blood outlet and a second Doppler site in fluidcommunication with an outlet of the arterial filter.
 6. The system ofclaim 1, further comprising an air inlet in fluid communication with thesweep gas inlet.
 7. The system of claim 1, further comprising a flowcontrol configured to receive the oxygen source and in fluidcommunication with the vaporizer.
 8. A system for cardiopulmonarybypass, comprising: an oxygen source comprising a pressure regulatorconfigured to regulate an oxygen pressure; an air source; a flow controlconfigured to receive the oxygen source and the air source; a vaporizerin fluid communication with the flow control; a sweep gas reservoir influid communication with the vaporizer; an oxygenator fluidly connectedto a pressure regulator of the sweep gas reservoir via a sweep gasinlet, wherein the sweep gas inlet is configured to have asubatmospheric pressure and the oxygenator is configured oxygenate ablood; a vacuum regulator fluidly connected to the oxygenator via answeep gas outlet, and configured to provide the subatmospheric pressure;and a flow restrictor fluidly connected to the sweep gas inlet andconfigured to allow for a pressure drop from the oxygen source to theoxygenator, and disposed between the vaporizer and the oxygenator. 9.The system of claim 8, wherein the oxygenator is a microporous membraneoxygenator with a sealed housing.
 10. The system of claim 8, furthercomprising a control panel configured to operatively control parametersof at least one system component.
 11. The system of claim 8, wherein thesweep gas reservoir is configured to allow the vaporizer to operate atatmospheric pressure.
 12. The system of claim 8, further comprising acardiopulmonary bypass reservoir; a pump in fluid communication with thecardiopulmonary bypass reservoir configured to provide pressure to thesystem; an arterial filter fluidly connected to a blood outlet of theoxygenator and to the cardiopulmonary bypass reservoir; and a patientinterface fluidly connected to the arterial filter and to thecardiopulmonary bypass reservoir.
 13. The system of claim 8, furthercomprising a cardiopulmonary bypass reservoir; a pump in fluidcommunication with the cardiopulmonary bypass reservoir configured toprovide pressure to the system; a patient reservoir in fluidcommunication with a blood outlet of the oxygenator; a second pump influid communication with the patient reservoir configured toadditionally provide pressure to the system; and a patient simulatorconfigured to introduce carbon dioxide into the blood and remove oxygenfrom the blood, wherein the patient simulator is fluidly connected to acardiopulmonary bypass reservoir.